try/catch

Learn to catch errors with try/catch.

try/catch

An operation can throw an error; for example int.parse throws a FormatException when given a non-numeric string. To catch it you put the risky code inside a try block and the recovery code inside a catch block.

Example: try { final n = int.parse("apple"); } catch (e) { print("not a number: $e"); }. When an error occurs the program does not stop; the catch block runs and the program continues. The e variable carries the thrown object. This lets you show a friendly error message to the user.

The catch parameter

Inside catch (e) the variable e carries the thrown object. It is usually an Exception or an Error. You can get its text with e.toString(); print(e) does the same.

If you also want the stack trace you add a second parameter: catch (e, s). Here s is a StackTrace that shows where the error came from. Very useful when debugging. You can also write catch with no parameter: catch { ... }; then you cannot access the object but you still catch the error.

Exercise Your turn: catch a parse error

Inside a try block call int.parse on a non-numeric string. In a catch (e) block print "not a number". The program shows a friendly message instead of crashing.

Show example solution

void main() {
  try {
    int.parse("apple");
  } catch (e) {
    print("not a number");
  }
}

Test yourself

What does int.parse("apple") throw?
```
try {
  int.parse("apple");
} catch (e) {
  print(e);
}
```
- FormatException
- TypeError
- RangeError
- It does not throw
Show answer

FormatException — int.parse throws a FormatException on a non-numeric string.

The catch block

try {
  int.parse("apple");
} ____ (e) {
  print("not a number");
}

Show answer

try {
  int.parse("apple");
} catch (e) {
  print("not a number");
}

Match try/catch

try

holds risky code
catch

catches the error
e

the thrown object
FormatException

int.parse error

Next lesson: throw

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